1. Field of the Invention
This invention relates to surface acoustic wave (SAW) monolithic elastic convolvers (MEC) for forming a correlation between two acoustic signals propagating under an acoustic waveguide.
2. Description of the Prior Art
Monolithic elastic convolvers, which are well known in the art, form a correlation between two acoustic signals propagating under an acoustic waveguide. The acoustic waveguides in practice are formed from certain known metals on a substrate of (YZ) LiNbO.sub.3. The acoustic waveguide which also functions as a convolution plate tends to be from an inch to several inches long and at the output frequencies of these devices, typically 500-800 MHz, the convolution plate is comparable to an electromagnetic wavelength on the convolution plate. The problem for the convolver designer is to sum all of the signals present on the LiNbO.sub.3 convolving plate without suffering losses of signal strength due to phase delays in the output circuit and without inducing standing waves on the convolving plate microstrip line by the application of taps which introduce impedance mismatches in the microstrip.
In prior art approaches to this problem, the output microstrip (convolving plate) of the MEC has been bonded out to an output coaxial line and all signals have been allowed to propragate to the output port. With this technique, there is a loss of spatial uniformity due to the reflections that are induced by the impedance mismatch. The convolving plate has also been segmented and each segment bonded out to a summing node which connects to the output coaxial line. With these two approaches there is difficulty in achieving spatial uniformity and in summing the contributions from the various segments equally due to the inductance of the bond leads and due to the absence of electromagnetic impedance matching. In some prior art, attempts have been made to minimize the spatial non-uniformity in the convolving plate output by loading the ends of the waveguide with resistors that are matched to the impedance of the waveguide. This technique eliminates reflections from the ends of the convolving plate and thus promotes spatial uniformity, however, it has the disadvantage of dissipating output energy. Alternatively, there have been efforts to tune the ends of the waveguide with an inductor to achieve a compromise between spatial uniformity and output signal strength. Again, with this approach the spatial uniformity is not as good as is desired.